1. Field of the Invention
The present invention relates to a code division multiple access(CDMA) mobile communication system, and more particularly to a channel structure with burst pilots in reverse links of such a CDMA mobile communication system. In other words, the present invention concerns a method for transmitting burst pilots from a plurality of mobile stations to a base station through respective reverse links.
2. Description of the Prior Art
Conventional digital CDMA cellular systems(DCCS's) have forward traffic channels which are classified into four kinds of channels, namely, the pilot, synchronization, paging and traffic channels.
The pilot channel serves to make the mobile station associated therewith catch the timing of the forward CDMA channel. The pilot channel also provides a reference phase for coherent modulation. In order to determine the point in time at which a hand-off occurs, the pilot channel provides means for comparing the signal strength of the associated base station with that of another base station. In addition, the pilot channel performs the transmission of pilot signals which are direct sequence spread spectrum signals continuously transmitted by each CDMA base station without being modulated.
The synchronization channel provides signals synchronous with mobile stations.
The paging channel is a forward control channel. This paging channel is used in transmitting commands for calling mobile stations.
On the other hand, the traffic channel is a channel for transmitting data transmitted from or to the user between the user's mobile station and base station.
The forward link provides a signal being sent from the base station to the mobile station whereas the reverse link means a signal being sent from the mobile station to the base station.
As apparent from the above description, the forward link using the pilot channel utilizes a coherent demodulation receiving system. On the other hand, each base station is concentrated with pilots corresponding in number to the mobile stations associated with the base station through reverse links. For this reason, the reverse links must use a non-coherent demodulation receiving system using no pilot channel in order to avoid an overload caused by pilots as well as co-interference among pilots. If such reverse links use the coherent demodulation receiving system, the base station associated therewith may simultaneously receive pilot and traffic signals respectively transmitted from many reverse links, thereby causing it to overload. In this case, the pilot signals may interfere with one another.
The above-mentioned conventional forward and reverse links have used coherent and non-coherent demodulation receiving systems in accordance with the IS-95 standard scheme (namely, the digital cellular telephone standard scheme for the CDMA system), respectively. According to the IS-95 standard scheme, however, the reverse link has a degraded performance in comparison with the forward link. Furthermore, the reverse link requires a complex receiver construction. In spite of the complex construction, the receiver has a limited channel capacity (36 channels). As a result, the reverse link according to the IS-95 standard scheme involves a limitation on the total number of channels.
Therefore, it is required to provide a reverse link capable of having a channel capacity larger than that according to the IS-95 standard scheme while requiring a simple receiver construction.
Meanwhile, the coherent demodulation receiving system exhibits a superior performance in wideband CDMA personal communication services (PCS) as compared to the non-coherent demodulation receiving system because it uses pilots. This is because the non-coherent demodulation receiving system consumes a larger amount of signal intensity, that is power than the coherent demodulation receiving system to obtain the same voice signal. Accordingly, it is advantageous to use the coherent demodulation receiving system for the wideband CDMA PCS.
This will be described in detail. Forward and reverse links for the wideband CDMA PCS use the coherent demodulation receiving system using pilots. In this case, however, such pilot signals all interfere with other users (namely, mobile stations) even though there is an advantage when it is extracted from the associated reverse link. Therefore, it is necessary to provide a method for transmitting pilot signals through reverse links without interfering with other users.